Cycles GPU Render button is looking passive
From the manual:


  To enable GPU rendering, go into the User Preferences, and under the
  System tab, select the Compute Device(s) to use. Next, for each scene,
  you can configure to use CPU or GPU rendering in the Render
  properties.
  
  OpenCL
  
  OpenCL is supported for GPU rendering with AMD graphics cards. (We
  only support graphics cards with GCN architecture 2.0 and above). 
  
  If your video card is supported, download the latest drivers.